What Killed The Dinosaurs?

THE INVALID HYPOTHESES

There are almost as many hypotheses about what killed the dinosaurs as there are
researchers studying the topic. Anyone can make a hypothesis  come one, make one up
right now  what do YOU think killed the dinosaurs? Now, knowing what you do about what science is, evaluate that
hypothesis. Is it falsifiable? Has it been subjected to peer review? Is it replicable by another
scientist? Good. Now you're ready to examine what scientists have proposed over the
years. These little gems have not withstood the test of peer review well. We'll name a few,
describe them, and evaluate their scientific value.

Hay fever killed the dinosaurs? Flowering plants (the
angiosperms) arose in the late Cretaceous period. Dinosaurs died in the late
Cretaceous period. So dinosaurs died from allergic reactions to these new, poisonous
intruders. What's wrong? Unfalsifiable, and pretty silly. Flowering plants were
around for millions of years before the dinosaurs disappeared. We have no evidence
whatsoever that their pollen or other pathogenic products killed the dinosaurs, and it is
unlikely that we would find evidence if it existed. It still wouldn't explain the massive
marine extinction; there have never been marine angiosperms. Indeed, around the K-T boundary
there is the phenomenon known as the "fern spike": all land plants except ferns show a dramatic
decline in diversity. How could the dinosaurs be wiped out by pollen if a lot of the pollen
producers were gone too?

Sniffles killed the dinosaurs? Near the end of the Cretaceous period, the
continents were shifting, opening new routes for dinosaurs to cross into other areas
previously inaccessible to them. So the mingling dinosaurs spread diseases and wiped each
other out. What's wrong? Again, unfalsifiable, and pretty silly. We have no
evidence of widespread disease among the last dinosaurs; they seem quite normal (we
could prove disease in a dinosaur by the bone pathology  diseased animals often have
deformed, weakened bones). There is no conclusive evidence of mingling dinosaurs,
either. And again, what happened to the oceanic and non-dinosaurian critters?

Dinosaurs got so darned big that they crushed themselves? This one is
falsifiable, and we have done so. There is no general trend of increasing size among all
dinosaurs throughout the Mesozoic era; certain lineages
 for example, some (but not all) theropods
and ceratopsians do show conspicuous trends of
increasing size during their evolutionary history (an example of the evolutionary principle
called Cope's Law), but none became so large that they
couldn't move; that is an evolutionary impossibility. Many (in fact, most) dinosaurs were of
medium or small size, even at the end of the Cretaceous. So not all dinosaurs were big, and
none were too big. And finally, once more (say it with me), this hypothesis does not explain
the marine or non-dinosaurian extinctions.

Mammals outcompeted the dinosaurs? Better still, but not easily
falsifiable and not upheld by any evidence. We cannot reconstruct the late Cretaceous
ecosystem and see if the mammals were outcompeting the dinosaurs for food, space, or
other resources; the fossil record is too poor and does not preserve behavior well. We
know that dinosaurs and mammals evolved together for most of the Mesozoic era;
mammals remained quite small, and only slowly increased in diversity. If they were
outcompeting the dinosaurs, we would see a trend of decreasing dinosaur diversity and
increasing mammalian diversity. We don't. Mammals and dinosaurs probably did not
occupy similar ecological niches; the small mammals could exploit rare food
resources, while the larger dinosaurs could not survive eating the same things that the
mammals did. If they did not occupy similar niches, they probably did not compete. Finally,
mammals were not found in the oceans at that time, so the marine collapse lacks an explanation.

Mammals ate all of the dinosaurs eggs? Another unfalsifiable
hypothesis, no matter how convincing it may sound. Most dinosaurs did have eggs, it
seems, but we have no evidence of mammals eating them, even though some may well
have done so. Some theropods probably ate other dinosaurs' eggs.
But no egg-eaters could eat all of the dinosaurs' eggs; they would eat
themselves into extinction if they did (they would have
no more food). If a mammal changed its diet to eat eggs, it could not eat every single
dinosaur species (except birds) out of existence if it tried. Since there were no marine
mammals in the Cretaceous period, they couldn't have eaten all of the marine animals' eggs either.

Cosmic rays killed the dinosaurs? Not as silly as it may sound, but still
unfalsifiable. We know of no extraterrestrial events that were occurring at the K-T
boundary that could have emitted sufficient dangerous radiation to affect life on Earth significantly.
There is no evidence for irradiated dinosaurs, either; this would have to show up in the
bones to be noticed.

The dinosaurs just faded away into extinction? Now this is an actually
good, falsifiable science. It does have some merit to it 
there is an amount of evidence that does suggest a gradual decline in dinosaur diversity
in the late Cretaceous period  sauropod
dinosaurs and many others were already extinct at the end of the Cretaceous period, at
least in North America. However, this hypothesis runs into the problems of time resolution and the
inadequacy of the fossil record, so at this point it is not considered absolutely
true. Also, there is no causation implied here  the hypothesis simply states that dinosaurs
slowly went extinct. There is a better, more plausible alternative. Finally, the marine realm
shows less evidence of a slow decline.

You've seen some not-so-good hypotheses; continue to see the modern, more scientific
ones.